I. CAMERA FEATURESAND OPERATION 15
photographywhen another subject or obstacle temporarily blocks the original subject.
The defaultsetting is 0.5 seconds, but with this Custom Function it can be tweaked
faster or slower according to the photographer’spersonal preference.
•C.Fn 20-0 isthe camera’s standard setting. It programs the AF to remain at the most
recentlyfocused distance for up to 0.5 seconds while an obstacle is blocking the
originalsubject.
•C.Fn 20-1 (“Slow”) programs the AF to remain at the most recently focused distance
for up to 1 second while an obstacle isblocking the original subject.
•C.Fn 20-2 (“Moderately Slow”) programs the AF remain at the most recently focused
distance for up to 0.75 secondswhile an obstacle is blocking the original subject.
•C.Fn 20-3 (“Moderately Fast”) programs the AF to refocus on a new subject
approximately0.25 seconds after the original subject is no longer being tracked.
•C.Fn 20-4 (“Fast”) programs the AF to refocus on a new subject approximately 0.125
secondsafter the original subject is no longer being tracked.
In C.Fn 20-3 and 20-4, the lensfocusing motor may seem more “jittery” than usual as it
constantlyrefocuses the lens. This is normal.
Custom Function 21first appeared with the EOS-1D Mark II. It is the same as Personal
Function 13 from the EOS-1D and EOS-1Ds. The purpose ofthis setting is to control the
camera’sshutter release timing during continuous bursts in AI Servo AF. In all cases,
release timing for the firstshot is controlled by the photographer, and the camera will
fire whether or notthe subject is in focus. This is called “Release Priority,” as opposed
to “FocusPriority,” in which the camera does not fire until the AF system has confirmed
thatthe subject is in focus.
•C.Fn 21-0: FocusPriority within a prescribed time limit*for the second and
subsequentshots in a continuous burst with AI Servo AF.
* The shutter willrelease at the framing rate set by the photographer, aslong as lens drive can be completed
within a prescribed time limit. The shutter releasesat the end of the prescribed time limit, even if focus has
notbeen achieved.
•C.Fn 21-1: Release Priority for the second and subsequent shots in a continuous burst
with AI Servo AF.
Some photographersprefer C.Fn 21-1 because it ensures that the camera will fire at a
consistentframing rate, even though it means that the subject may not be accurately
focused. However, thisis clearly a “use at your own risk” scenario, because there isno
wayto be sure that your photos will be sharp.
PersonalFunction 14 disables focus detection (“hunting”) by the lens drive. Thiscan
save time in low lightor low contrast situations, especially when working with a USM
lensequipped with a distance scale. Such lenses can be focused manually even when
setto their AF mode.